One process in the production of coated conductors (superconductive tapes or films) has been referred to as a thick film process where the thickness of the superconductive layer is generally at least one micron in thickness. In the thick film process, it has been shown that YBCO thin films on single crystal substrates can achieve critical current density (Jc) values of over 106 amperes per square centimeter (A/cm2) at 77 K. The use of polycrystalline substrates other than single crystals was desired. For amorphous or polycrystalline substrates, the use of a suitable buffer layer to provide the necessary structural template was developed. For example, a YSZ buffer layer has been deposited by use of ion beam assisted deposition (IBAD) in which a YSZ layer is deposited in combination with irradiation from an ion beam directly on a substrate during the deposition. For example, both Iijima et al., U.S. Pat. No. 5,650,378 and Russo et al., U.S. Pat. No. 5,432,151 have demonstrated deposition of in-plane textured YSZ buffer layers with IBAD, leading to YBCO thin films having excellent superconducting properties. Arendt et al., U.S. Pat. No. 5,872,080 described a coated conductor having the structure YBCO/Y2O3/YSZ/Al2O3/Ni alloy with a high critical current density (Jc) of about 1×106 A/cm2 and a high transport critical current (Ic) of from about 100 to about 200 A/cm. The in-plane textured YSZ of Arendt et al. was more highly aligned than that obtained by either Iijima or Russo. While the current was satisfactory, the deposition of the YSZ layer was considered too slow for commercial production.
In WO 99/25908, thin films of a material having a rock salt-like structure were deposited by IBAD upon amorphous substrate surfaces. Among the materials with a rock salt-like structure was magnesium oxide (MgO). In comparison to the deposition of YSZ, MgO can be rapidly deposited (about 100 times faster) through an IBAD process. The structures of WO 99/25908 included, e.g., YBCO/Y2O3/YSZ/MgO/MgO(IBAD)/Si3N4/Ni alloy with a NiO layer in between the YSZ layer and the MgO layer in most instances. Despite the improvement in processing speeds, the structures of WO 99/25908 had Ic's of only about 50 to about 75 A/cm. In addition, at the elevated processing temperatures needed to form the superconductive layer, the silicon nitride layer reacts with other materials in the system.
Thus, further improvements in the structure and resultant properties of coated conductors have been desired. After extensive and careful investigation, improvements have been found in the preparation of superconducting films on polycrystalline substrates such as flexible polycrystalline metal substrates.
It is an object of the present invention to provide superconducting films, especially YBCO superconducting films, on polycrystalline substrates such resultant articles demonstrating properties such as high Jc's and Ic's.
It is another object of the present invention to provide structural template articles for subsequent deposition of oriented films, e.g., superconducting films, especially YBCO superconducting films.